1. Field of the Invention
The present invention is an endoscope system having a special observation mode, a processor device of the endoscope system, and a method of operating the endoscope system.
2. Description of the Related Art
In the medical field, for example, diagnosis using an endoscope system comprising a light source device, an endoscope, and a processor device has been widely performed. The endoscope system includes an endoscope system having a special observation mode in which an observation part inside a living body is irradiated with first illumination light and second illumination light having different spectral characteristics and observation is performed.
In the special observation mode, the first illumination light and the second illumination light are alternately supplied from the light source device to the endoscope, and an observation part is irradiated with the light from a distal end portion of the endoscope. For example, the first illumination light is white light (normal light), and the second illumination light is special light including light for which blood hemoglobin has a high light absorption coefficient. A normal observation image is generated by imaging an observation part illuminated by the first illumination light, and a special observation image is generated by imaging the observation part illuminated by the second illumination light.
In an endoscope system of the related art, a charge coupled device (CCD) type imaging element is used as an imaging element of an endoscope, whereas in recent years, a complementary metal-oxide semiconductor (CMOS) type imaging element is used (see JP2010-68992A). This is because the CMOS type imaging element has lower power consumption than the CCD type imaging element or peripheral circuits such as an analog-to-digital converter (ADC) circuit can be formed on the same substrate as that for the imaging unit. In this CMOS type imaging element, basically, a rolling shutter scheme in which resetting and signal reading are performed sequentially pixel row by pixel row for a plurality of pixel rows configured in the imaging unit is adopted. A period from the resetting of each pixel row to signal reading is an exposure period.
Since an exposure timing is shifted sequentially pixel row by pixel row in the rolling shutter scheme, assuming that illumination light is switched from the first illumination light to the second illumination light without interruption while driving the imaging element in the rolling shutter scheme, an exposure period of some pixel rows straddles switching of the illumination light, and a combination of the first illumination light and the second illumination light is imaged. Therefore, JP2010-68992A proposes that a turn-off period is provided in the event that the illumination light is switched, and signal reading is performed during this turn-off period.
As described above, assuming that a turn-off period is simply provided in the event that first illumination light and second illumination light are switched, a frame rate of the imaging element is decreased due to the turn-off period. Therefore, in JP2010-68992A, a decrease in the frame rate is prevented by shortening each irradiation time (exposure time) of the first and second illumination light and shortening a reading time by decimating the number of pixels on which signal reading is performed from the imaging element.
However, in the endoscope system described in JP2010-68992A, first illumination light is radiated at all times and the imaging element is driven in a rolling shutter scheme in a normal observation mode. However, in a special observation mode, since an exposure time is shortened so that the frame rate is not decreased, there is a problem in that luminance and a signal-to-noise (S/N) ratio are decreased in a normal observation image obtained in the special observation mode in comparison with a normal observation image obtained in the normal observation mode.